If Workspace.getIndexWriter(...) method is called and a LockObtainFailedException is thrown, the method getIndexWriter catch the LockObtainFailedException and returns null. This causes NPE in PerDPQueueProcessor.run() method (delegates expect that writer is not null) and writerLock is released by calling workspace.forceLockRelease() method because of tw.getCause() returns NullPointerException instead of expected LockObtainFailedException.